Organic EL display devices have organic EL elements and are arranged in a matrix as pixels and individually controls the emission of the organic EL elements of the respective pixels to make display. Organic EL display devices include an active type and a passive type. The active type organic EL display device, has associated with each pixel, a pixel, a drive circuit for controlling current through the corresponding organic EL element. Active matrix types of drives are better for high definition display.
FIG. 1 shows an example of the pixel circuit of an active type organic EL display device. A drive TFT 1 is a p-channel type and has a source connected to a power supply PVdd which extends in a vertical direction and a drain connected to an anode of an organic EL element 2. A cathode of the organic EL element 2 is connected to a cathode power supply CV.
A gate of the drive TFT 1 is connected to a source of an n-channel type selection TFT 3. A drain of the selection transistor is connected to a data line Data which extends in a vertical direction, and a gate thereof is connected to a gate line Gate which extends in a horizontal direction. The gate of the drive TFT 1 is also connected to one end of a retention capacitor C, the other end of which is connected to a capacitor power supply Vsc.
Thus, the selection TFT 3 is turned on when the gate line Gate is set to a high level. At this time, when an image signal representing luminance of the pixel is applied to the data line Data, a voltage of the image signal is held in the retention capacitor C and applied to the gate of the drive TFT 1. Therefore, a gate voltage of the drive TFT 1 is controlled by the image signal, and the current flowing to the organic EL elements 2 is controlled. The gate voltage of the drive TFT 1 is held by virtue of the retention capacitor C even after the selection TFT 3 is turned off.
A luminous volume of the organic EL elements 2 is substantially proportional to its drive current. Therefore, the organic EL elements 2 emit light according to the image signal.
Here, the display device does not have a linear relationship (gamma) between the input signal level and the display luminance. Therefore, gamma compensation is performed in order to provide an appropriate relationship. To turn on the drive TFT 1 in the pixel circuit shown in FIG. 1, a gate-to-source voltage Vgs must be a prescribed threshold voltage (Vth) or more. The image signal is basically data corresponding to the luminance of emitted light, and the minimum level corresponds to the black level. Therefore, a data voltage to be supplied to the pixel circuit is required to carry out black level offset setting so to offset the image signal by a voltage corresponding to the threshold voltage Vth.
FIG. 2 shows an example of a conventional structure to perform black level offset setting and gamma compensation. Respective signals of RGB are gamma-compensated by respective gamma compensation lookup tables (LUT) 5R, 5G, 5B, converted into analog signals by D/A converters 6R, 6G, 6B and input to a display panel 7. Contents of the LUTs 5R, 5G, 5B have the table data stored in the flash memory 9 written therein by a CPU 8 before they are shown on the display panel 7.
In this example, the black level offset is adjusted by the D/A converters 6R, 6G, 6B so that a black input voltage of the display panel 7 can be output for the black signal. Since the contents of the LUT 5R, 5G, 5B are adjusted to values so that the black input voltage of the panel is output for the black signal, it is possible to omit adjustment by the D/A converters 6R, 6G, 6B.
In any event, optimum gamma table values and black level values of each color must be measured for each panel before shipping from a factory. When the TFT characteristics are substantially the same for the respective colors, the black level offset voltage value may be common to RGB.
To measure the gamma and black level offset voltage of the display panel 7, data to provide a linear input/output characteristic is written in the LUTs 5R, 5G, 5B, and the input signal is changed in this state to measure the luminance characteristic of the panel for the respective colors RGB.
A gamma compensation circuit for a display is proposed in Japanese Patent Laid-Open Publication No. Hei 6-245222 (hereinafter referred to as the patent publication 1) or the like, but it does not propose what gamma compensation is performed for the organic EL panel.